Eyes that noticeably show the whites around all or most of the iris. Can indicate craziness, shock, surprise, anger, exhaustion, or a dangerous demeanor. Sometimes referred to as "Dead fish eyes" among western speakers.
The Japanese expression literally translates to "three white" as in the whites of the eyes being visible to at least three of four sides of the iris. Probably derives from the fact that when in an alert excited state, or in shock, people often open their eyelids wide, and have constricted pupils.